This invention relates to biomaterials useful in bone repair and replacement, especially as used for orthopedic, dental and oral surgery. More particularly, this invention relates to biomaterials having a special surface which resorbs more slowly than the underlying base.
Porous carbonate echinoderm or scleractinian skeletal material of marine life has a unique structure. This material has a uniformly permeable interconnected three dimensional porosity characterized by a substantially uniform pore volume in the range from about 10 to about 90%. The microstructure of this material resembles the cancellous structure characteristic of bony tissue or bone. Because of this unique microstructure of the porous carbonate echinoderm or scleractinian coral skeletal material of marine life, these materials are useful as bone substitutes. However, the carbonates of this material, such as provided in echinoid spine calcite and Porites skeletal aragonite, do not have the desired durability for use as bone substitutes.
A technique has been developed to convert the foregoing calcium carbonate coral materials to hydroxyapatite while at the same time retaining the unique microstructure of the coral material. U.S. Pat. No. 3,929,971 (incorporated herein by reference) discloses a hydrothermal exchange reaction for converting the porous carbonate skeletal material of marine life into a phosphate or hydroxyapatite skeletal material possessing the same microstructure as the carbonate skeletal material. These synthetic hydroxyapatite materials have been produced commercially and are available from Interpore International Inc., Irvine, Calif., under the tradename Interpore-200, which is derived from certain coral of the genus Porites, which have an average pore diameter of about 200 microns, and under the tradename Interpore-500 derived from certain members of the family Goniopora, which have pore diameters of about 500 microns.
Interpore-200 and Interpore-500, have also been identified as replamineform hydroxyapatite and coralline hydroxyapatite, have been found to be useful as bone substitute materials in dental and surgical applications. These materials are essentially nondegradable. More information concerning these materials can be found in the article by Eugene White and Edwin C. Shors entitled "Biomaterial Aspects of Interpore-200 Porous Hydroxyapatite", which appeared in Dental Clinics of North America, Vol. 30, No. January 1986, pp. 49-67, incorporated herein by reference.
However, while calcium phosphates such as Interpore-200 and Interpore-500 are satisfactory for many applications, and promote the ingrowth of bone and other tissue into and around the implant, they do not satisfy all of the needs of surgeons using them as bone replacements or implants.
For some applications, surgeons prefer that bone substitutes resorb within a few weeks or months following implantation, after new bone has grown through the implant site. One approach to increase the degradation rate of ceramic implants has been to use tricalcium phosphate instead of hydroxyapatite. Tricalcium phosphate degrades, but its rate of degradation is inconsistent and unpredictable. Another approach utilizes polymers that are biodegradable and non-toxic to the host into whom the polymer is implanted. However, there is little evidence that these materials are osteoconductive or have adequate interconnected porosity.
Accordingly, it is an object of this invention to provide a ceramic biomaterial which degrades in a predictable manner and at an acceptable rate.
It is another object of this invention to provide bone substitute materials and methods for their manufacture derived from solid or porous calcium carbonate and having a surface layer of hydroxyapatite.
It is a further object of this invention to provide bone substitute materials derived from coral having the unique porous microstructure thereof, while having a more slowly resorbing layer of calcium phosphate or hydroxyapatite.
It is a still further object of the invention to provide bone substitute materials which include a calcium phosphate layer throughout the porous structure of coral without compromising the porosity of the structure or its interconnectedness.
It is another object of the invention to provide solid or porous calcium carbonate granules having calcium phosphate surface regions.
It is a further object of the invention to provide a degradable biomaterial which provides an adherent surface for growth factors and antibiotics.
How these and other objects of this invention are achieved will become apparent in light of the accompanying disclosure.